At present, due to continuous increase of the speed of the high speed rail, a large challenge is brought to the performance of base station handover of the existing vehicle-ground communication system. First of all, Doppler frequency offset and channel fast fading due to high speed movement bring a large threat to the high speed data transmission. The channel will present fast time-varying characteristics, which is particularly obvious in a Time Division Duplexing (TDD for short) system, and may possibly results in a base station being difficult to demodulate a received signal. At the same time, in a handover area, a train will receive signals from two adjacent base stations, which have close energy and Doppler frequency offsets of which one is two times larger than the other, and the signals have a positive and negative frequency offset jump characteristic. Secondly, the increase of the speed will results in a problem of frequent handover, which raises higher demands on the system mobility management (particularly fast handover control). Finally, as the geographical positions of passengers on the train are relatively centralized, when the train passes across a cell edge, cluster handover will occur. This is a problem which is rarely faced in the traditional communication system.
In a wireless communication network, the base station handover is primarily divided into two kinds, i.e., hard handover and soft handover. In a Long Term Evolution (LTE for short) network, on one hand, as a Radio Network Controller (RNC for short) node in the 3G network is cancelled, the handover is processed by the base station (eNB), and the network loses a centralized control capability; and on the other hand, the LTE is an Orthogonal Frequency Division Multiplexing (OFDM for short) system and uses co-frequency networking, this results in a User Equipment (UE for short) in the LTE system not being able to receive signals of two adjacent cells at the same time on the same frequency like the UE in the 3G system. Therefore, the LTE can only use a manner of hard handover. Due to the characteristics of “break-before-make” for the hard handover, interruption necessarily occurs in the handover process, which has a serious effect on the handover performance.
In the related art, Coordinated multi-point (CoMP for short) refers to a technology that multiple transmission points separated geographically participate in coordination in transmitting data to a terminal or in coordination in receiving data transmitted by a terminal. According to different transmission schemes, the CoMP is divided into two types, i.e., joint processing and coordinated scheduling/beam-forming. The joint processing is further divided into two types, i.e., Joint Transmission (JT) and dynamic cell selection, wherein, the primary principle of the CoMP-JT technology is that multiple cell base stations participating in coordination transmit the same data to a terminal at the same time on the same time-frequency resources through a Physical Downlink Shared Channel (PDSCH for short), so as to convert inter-cell interference signals in the conventional LTE network into useful signals, thereby achieving the purpose of reducing the interference and increasing the reception power, and effectively obtaining joint processing gain and diversity gain. The CoMP was originally proposed to solve the interference to users at the cell edge, so as to increase the throughput of the users at the edge. However, it can be seen from the principle of the CoMP-JT that it can fundamentally solve the problem of LTE hard handover, thereby achieving a basic requirement of constituting soft handover, and thus improving the handover performance.
In addition to the handover success rate, the handover delay is also an important index to be considered in the handover process. In the conventional LTE handover process, a “data forwarding” mechanism of a Universal Mobile Telecommunication system (UMTS for short) continues to be used, and in this process, there is a degree of eNB processing delay and X2 transmission delay. LTE is an IP-based transmission network, which may achieve data sharing between a serving base station and a target base station using the existing “bi-casting” technology in the IP network, that is, a Service GateWay (SGW for short) transmits the received original data to the serving base station and the target base station to be handed over on two paths without any change. In handover process, there is a short interruption time for the “bi-casting”. However, if the conventional “bi-casting” mechanism is directly introduced into the LTE handover, a problem that lossless data transmission cannot be ensured may occur. When the “bi-casting” starts, a condition that the serving eNB cannot correctly transmit a packet to the UE may possibly already occur, and if the target eNB directly forwards a data package from the SGW after the UE successfully accesses, theses lost packets may be beyond retrieval. At the same time, if the “bi-casting” manner is used, the complexity of the system will increase to some extent.